1. Field of the Invention
The invention relates to navigation systems particularly with respect to multiple mode, multiple sensor area navigation systems (RNAV) for aircraft.
2. Description of the Prior Art
Navigation systems for aircraft traditionally utilize a plurality of navigation sensors (navaids) and operate in a plurality of navigation modes (navmodes). For example, radio aids such as VOR and DME for airport-to-airport navigation and localizers for terminal guidance are utilized. The aircraft is often equipped with onside (leftside) radio receivers and offside (rightside) receivers for these radio navaids. Additionally, the navigation system aboard present day aircraft often includes an inertial navigation system (INS) or an inertial reference system (IRS).
Such navigation systems operate in a plurality of modes variously utilizing the navaid sensor complement. Such traditional modes include RHO-RHO, BEELINE, RHO-THETA LEFT, RHO-THETA RIGHT, LOCALIZER LEFT, LOCALIZER RIGHT, INS and DEAD RECKONING. In RHO-RHO and BEELINE navigation, the aircraft position is determined utilizing two DME distances where each DME sensor provides range from a known reference point. In the RHO-THETA LEFT navigation mode, the aircraft position is generated utilizing the VOR (VHF Omni Range) bearing and DME (Distance Measuring Equipment) distance from a known reference point, the bearing and distance data being provided by the leftside VOR and DME sensors. The RHO-THETA RIGHT navigation mode is similar to the RHO-THETA LEFT mode except that the bearing and distance data is provided by the rightside VOR and DME sensors. In the LOCALIZER modes lateral displacement from a localizer beam is provided by beam error data from the onside or offside localizer in accordance with the navigation mode utilized. In the INS navmode, the aircraft position is set equal to the position output of the INS. In the DEAD RECKONING mode, the aircraft position is set equal to the position determined from the DEAD RECKONING algorithm utilized.
Such multiple-mode, multiple-sensor navigation systems traditionally select the sensors and modes to be utilized in position computations in accordance with a predetermined procedure. In the prior art, a fixed hierarchy of priorities is established for navigation mode selection and sensor usage. For example, in an aircraft navigation system, having two VOR/DME sensors and one inertial navigation unit, the hierarchy of navigation modes and sensor usage might be established in the following order: RHO-RHO, RHO-THETA LEFT, RHO-THETA RIGHT, and INS.
For each navigation mode and sensor combination, a set of minimum requirements must be fulfilled before the mode is effectuated. For example, RHO-THETA LEFT navigation cannot be enabled without valid bearing and distance data from the leftside colocated VOR/DME navaid. Essentially, in prior art multiple-mode, multiple-sensor navigation systems, the navigation mode and sensor combination having the highest priority for which minimum requirements are fulfilled is selected. It was assumed in the prior art that the fixed hierarchy selection would result in the navigation mode and sensor combination providing the optimum position estimate. It was determined, however, that under commonly occuring conditions, this assumption is incorrect whereby inaccurate navaid data is utilized even though more accurate data is available. For example, the performance of the system described above, in a typical departure scenerio for a commercial airliner is considered. Immediately prior to take-off, the flight crew aligns the INS. Shortly after the aircraft becomes airborne, the leftside VOR and DME sensors receive valid radio navigation distance and bearing data from a single distant VOR/DME station. Under the fixed hierarchy described above, the RHO-THETA LEFT mode is selected as the highest priority navigation mode. This is undesirable because RHO-THETA navigation performed using a distant VOR/DME station typically results in a position computation that is less accurate than the position provided by a recently aligned INS. The INS, however, tends to drift with time. It is very accurate at the beginning of a flight, but tends to become increasingly more inaccurate as time increases. Therefore, if a long period of time has elapsed since the INS was last aligned or corrected, resulting in a degredation in the accuracy of the INS system, RHO-THETA navigation may or may not provide a more accurate position estimate at that time. Additionally, if for example, the prior art system is performing RHO-RHO navigation when criteria for RHO-RHO navigation can no longer be fulfilled, the system reverts to RHO-THETA navigation providing RHO-THETA criteria can be met. However, under these conditions, a more accurate position estimate is achievable by utilizing the radio corrected INS or DEAD RECKONING, for at least a short time, from the last RHO-RHO computed position.
These shortcomings of the prior art fixed hierarchy procedure result from the failure to make optimum usage of the navigation modes and sensors in the sense of minimizing the value of estimated position error. The fixed hierarchy procedure does not take into account that the quality of a particular navigation mode and sensor combination is not static but varies dynamically as a function of time, position and other variables. Thus, prior art navigation systems compute aircraft position utilizing inaccurate radio navaid data, even when an on-board inertial navigation system is providing a more accurate position.